Can someone explain how it works....

Does Inv Toughness or Imperv reduce it?

I was once told that it punched through those, but the FAQ says that Toughness and others are resolved against it, so I'd say that yes, Poison is reduced by Toughness, Invulnerability, and Impervious

Toughness and Invulnerability have a clause allowing them to reduce damage from superpowers, and so will work against Poison. Impervious requires that the figure be targeted by an attack, and so Poison will ignore Impervious.